There were some 18" 5-spoke chromes on e-bay just a few days ago. Check there. You can get stock wheels dirt cheap on e-bay. Otherwise you will pay almost as much as you would for aftermarket wheels, in which case you might as well get better wheels for the same price.
But like I said, e-bay is reeking of used 18's for the Z, standard 6 spoke, chrome 5 spoke, and anniversary editions can be had. Also track model 18's (MUCH better than the other stock wheels for the Z).
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